Since the 1950's, pelletizing has been widely practised as a method of agglomerating iron ore fines as an aid to materials handling and transport. The first large plant in the United States with a capacity in excess of 6 million tons per annum commenced operation in 1955 and by 1975 iron ore pellet production in the United States was almost 60 million tons per annum. Pelletizing plants are now operated in many countries including Australia to facilitate intra and international transport of iron ores and their use in steel making.
The most widely used process is know as the "Fired Pellet", "Indurated" or "Hot Bonding" process and consists of two distinct operations; forming pellets at atmospheric temperature and then firing them at high temperature which is meant a temperature in the region of 2350.degree. F. (1300.degree. C.). In the first operation the pellets are first formed by rolling moist fine ore in either a horizontal drum, or more usually an inclined disk, to form spheres known as "green balls" or pellets. At this stage the pellets are required to have adequate strength to withstand handling to the firing stage and sometimes a minimum quantity of an inorganic or an organic binder is added to assist in achieving such adequate strength. In the second operation the pellets are fired. Firing is normally carried out in shaft furnaces, rotary kilns or travelling grate furnaces using gas or oil as fuel. The resulting fired pellets are typically spheroidal, of approximately 14-15 mm diameter, and have a cold compression strength of approximately of 500 lbs. force per pellet. "Cold compression strength" as herein used in a term familiar to those skilled in the art and is a measure of the load required to be applied to cause crushing of a substantially spherical pellet of predetermined diameter placed between two plates of an Instron tester or the like. As such they may be repeatedly bulk handled for example in loading to railway cars, unloading of railway cars, at port facilities, at steel works and the like, and withstand the rigours of transportion in bulk.
By virtue of the scale of throughput required by the industry, the capital investment in equipment necessary for the firing stage of pelletizing is major, and the energy consumption is high. The fuel costs of operating a typical installation are said to total millions of dollars per annum.
Many attempts have been made to find alternative agglomeration processes and in particular processes which would avoid the necessity for firing. To date no such process has proved satisfactory. The general requirements for good quality agglomerates include sufficient strength for handling and for long distance transportation, the ability to withstand outside storage without substantial detrimental effects, and complete reducibility in iron and steel furnaces without premature degradation or excessive swelling.
Numerous so called cold bonding processes have been developed in which inorganic binders such as cement, lime, magnesia, clays, bentonites and the like, or organic binder materials such as tars, petroleum residues, waxes, flours, paper industry by-products and polymers have been employed. In general the resulting pellets have been either too expensive to produce and/or have been deficient in strength, impact resistance or abrasion resistance. None of the organic and few of the inorganic binder cold bonding processes have warranted commercial production usage and of the latter none has achieved widespread acceptance.
An objective of the present invention is therefore to provide a method for pelletizing mineral fines which avoids the necessity to fire the pellets at high temperature while producing pellets of sufficient strength to permit handling, transportation and use in the manner usual for fired pellets.
A further objective is to produce pellets which, while not fired, are commercially acceptable as substitutes for pellets produced by the Fired Pellet process. To be commercially acceptable the pellets should be cost competitive, desirably have satisfactory strength, for example a compressive strength of the order of 300 lbs. force per pellet or higher, and for preference should be capable of manufacture in existing green ball forming equipment, ideally without significant modification of the operating conditions thereof.
Those objectives are achieved in preferred embodiments of the present invention by addition of a raw starch to the ore fines, pelletizing the resulting mixture substantially in accordance with the normal procedure for forming green balls, and subsequently gelatinizing the starch in situ.
Hitherto raw starch has sometimes been added to ores prior to pelletizing as a binder. As previously indicated binders may be added during the first stage of the fired pellet process for insuring that the green pellets have sufficient strength and abrasion resistance to withstand handling to the firing stage. However the resulting green balls have had a compressive strength typically of 10 lbs. that is to say one twentieth that typical of fired pellets, and have no merchantable utility until fired at 1200.degree. C. In some cases binders consisting of bentonite and/or clays have employed minor amounts (for example one quarter percent by weight of iron ore) of gelatinized starch. However the gelatinized starch was then present for the purpose of lubricating the binding composition and those binders were used prior to firing rather than as a substitute for firing. It has not previously been proposed to use gelatinized starch as a binder instead of firing or to gelatinize starch in situ.